This invention relates to determining film thickness of semiconductor materials. The very thin semiconductor films usually introduce more interference patterns in deep ultraviolet region than in visible and infrared regions. In other words, the reflected light wave patterns in either visible or infrared regions interfere destructively because the interferences become out of phase with each other as the wavelength increases.
Moreover, ultraviolet light penetrates materials because ultraviolet light possesses higher photon energies than visible and infrared lights. The light waves in UV region add up constructively even for very thin films. In addition, some films which present an opaque characteristic in the visible and near infrared regions pose a non-opaque characteristic in UV region. There are significant absorption lines in deep UV and UV regions which are useful for material characterizations. This information in conjunction with data from visible and near infrared data enables us to observe optical characteristics of materials as well as very thin film thicknesses valuable to semiconductor applications. To obtain data and to preserve the integrity of data in UV-VIS-IR spectrum a very accurate spectrophotometer design is needed.
Diode array spectrophotometer is specially fit to obtain high quality data. The diode array spectrophotometer is considered to have reversed optics in that all wavelengths of light excite the sample and is considered to be non-conventional compared to forward optics spectrophotometer wherein filtered monochromatic light is employed. Generally, a polychromatic light from a polychromatic source excites the sample under test. The reflected light passes through a focusing lens system which will focus the light onto the entrance slit of spectrograph. The incident and reflected light have to have a close to normal incident angle to preserve data integrity. Moreover, the light beam has to be focused onto the sample and then onto the entrance slit of spectrograph while optical losses are minimized. In other words, the light beam has to be configured such that the spot size onto the sample is minimum while optical losses are optimum. The optical lense configuration must provide a large depth of field such that the sample can exercise a noticeable degree of freedom in the Z-axis to encompass the mechanical vibrations of wafer handler in vertical direction. In lieu of above objectives and the fact that data acquisition has to be preformed in a speedy manner, the present invention provides an improved reflectance measurement system which covers UV-VIS-IR spectrum and preserves the date integrity.